Consistency, accuracy, no swabbing between shots, no crud ring, bullets load easily, either measure by volume or weight because the powder itself is the same consistant size, clean up time is at a minimal when using Hoppe's #9.
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Reasons stolen from another but, all the same reasons, i use it!! I never have had an issue with my breach plug, either. i let it soak in T17 while i clean the gun and then just a little cleaning with a brush and she is good. the ONLY negative i can come up with is price and even that is reasonable to me for what i get.